The invention relates to a method of preparing an apparatus for material processing by generating optical breakthroughs in or on an object, which apparatus comprises a variable, three-dimensionally acting focus adjustment device for focusing pulsed processing laser radiation on different locations in or on the object, wherein a contact element to be placed on the object is mounted to the apparatus, said contact element being transparent for the treatment laser radiation and comprising, on its side to be placed on the object, a contact surface and, located opposite, an entry surface for the processing laser radiation, each of said surfaces having a previously known shape, wherein, prior to processing the object, the position of the entry surface or contact surface with respect to the focus adjustment device is determined by irradiation of measurement laser radiation on said surface, by focusing measurement laser radiation near or on said surface by means of the variable focus adjustment device, with the energy density of the focused measurement laser radiation being too low to generate an optical breakthrough, and the focus position of the measurement laser radiation is adjusted in a measurement surface intersecting the expected position of said surface.
The invention further relates to a material processing apparatus which comprises a processing laser providing pulsed treatment laser radiation, an optical device for focusing the processing laser radiation into or onto an object to be processed, such that optical breakthroughs form in the focus, a focus adjustment device for variable adjustment of the focus position in or on the object, a contact element to be placed on the object, which contact element can be mounted to the apparatus and comprises a contact surface to be placed on the object and, opposite the contact surface, an entry surface for the treatment laser radiation, each of said surfaces having a previously known shape, and a control device for determining the position of the entry surface or of the contact surface after mounting the contact element and before processing the object, which control device controls the processing laser and the focus adjustment device, there being provided also a measurement laser radiation source, also controlled by the control device, for emission of measurement laser radiation, whose measurement laser radiation passes through the focus adjustment device and the optical device and causes no optical breakthroughs in the focus, wherein the control device, in order to determine the position of the surface, adjusts the focus of the measurement laser radiation in a measurement surface intersecting the expected position of the surface.
In material processing, a laser processing device is often employed to scan the regions of the object which are to be processed by a processing laser beam. The accuracy of positioning the laser beam usually determines the precision achieved in processing. If the laser beam is focused into a processing volume, exact three-dimensional positioning is required. Therefore, for high-precision processing it is usually indispensable to keep the object in an exactly defined position to the laser processing apparatus. For such applications, the aforementioned contact element is used, because it allows fixation of the object to be processed, whereby defined relationships are achievable up to the processing volume. In this way, the contact element becomes part of the beam path of the processing laser radiation.
This is required, in particular, in micro-processing of materials having only low linear optical absorption in the spectral range of the processing laser radiation. For such materials, non-linear interactions between the laser radiation and the material are used in the art, generally in the form of an optical breakthrough being produced in the focus of high-energy laser beams. Since the processing effect then only takes place in the laser beam focus, exact three-dimensional positioning of the focal point is indispensable. Thus, in addition to a two-dimensional deflection of the laser beam, an exact depth adjustment of the focus position is required. The contact element serves to ensure constant optical conditions, which are known with a certain accuracy, in the beam path to the object by mechanically coupling the object and the laser processing device and by also imparting to the object surface a shape having a known optical effect.
A typical application of such a contact glass is the ophthalmic surgery method known as femtosecond LASIK, wherein the laser processing apparatus, provided in the form of a therapeutic instrument, focuses a laser beam into the cornea, forming a focus with dimensions on the order of one micrometer. Then, a plasma forms in the focus, causing a local separation of the corneal tissue. By suitable sequential arrangement of the local separation zones thus generated, microscopic cuts are realized, e.g. a defined partial volume of the cornea is isolated.
The position of the contact element influences the accuracy of this method and, therefore, it is dealt with many times in the literature with respect to position determination:
U.S. Pat. No. 6,373,571 discloses a contact lens provided with reference marks. Said contact lens is adjusted by means of a separate measurement device, resulting in a relatively complex configuration. A further example of a contact element is described in EP 1,159,986 A2. It resembles the contact lens of U.S. Pat. No. 6,373,571, but additionally comprises a periphery in the form of a holder having line marks which enable a surgeon to visually position the device. However, such positioning is usually too inaccurate.
Since the contact element usually contacts the object to be processed, it is generally required to employ a new, separate adapter for each object. This applies, in particular, in ophthalmic applications under the aspect of sterility. As a consequence, prior to each processing operation or prior to surgical intervention, respectively, the contact element has to be mounted to the laser processing device, which is then provided e.g. as a therapeutic instrument. WO 03/002008 A1 teaches to mount the contact glass by holding it in a pincer-like means which is locked to the laser processing device. Said locking is effected via a collar guided within a rail. The adapter is pushed in transversely to the optical axis in a form-locking manner. DE 19831674 A1 describes the use of a mechanical coupling mechanism wherein a metal rod, attached to a mount of a contact glass at an oblique angle, is held in a sleeve by means of a magnet or electro-magnet. However, these attachments do not define the position of the contact element with sufficient accuracy.
It is further known from WO 05/039462 A1 to provide a contact glass with position marks and to add to the laser processing device a confocal detector unit which, in connection with irradiated measurement laser radiation, allows to detect the position of the marks and to determine the position of the contact glass therefrom. The accuracy with which the position of the contact glass is known is thus better than the accuracy given by the mounting mechanism as described in DE 10354025 A1, for example, and by the manufacturing tolerances of the contact glass.
WO 04/032810 A2 also pursues the goal of accurately determining the position of the contact glass. It describes a method or a device of the type described above. For exact determination of the position of the contact surface of the contact glass, which surface is pressed onto the eye, said publication suggests to use the effect of the treatment laser on the contact surface. The treatment laser is controlled such that it is focused at a multiplicity of points and emits treatment laser radiation pulses. Those laser beam pulses which are focused on the boundary surface of the contact glass produce an optical breakthrough by a non-linear effect, which manifests itself as a corresponding plasma spark. Thus, detection of a spark gives an indication that the boundary surface is located at the present focus position of the treatment laser beam. The determination of a sufficient number of such points, which may be arranged in a plane perpendicular to the optical axis of the treatment laser beam, for example, is then used to determine the position of the contact glass. In an alternative approach, said publication suggests to utilize non-linear effects on the contact glass, which are produced by lower energy levels of the treatment laser pulses, i.e. by energy levels causing no optical breakthroughs. Of course, such non-linear effects below the energy threshold for optical breakthroughs occur only with certain contact element materials. The publication mentions non-linear effects in the form of the second harmonic of the irradiated treatment laser radiation or white-light radiation. The measurement or detection of such radiation poses high chromatic demands, because radiation, e.g., of twice the irradiated frequency has to be detected. This makes the optical system complex.
In a third approach, the aforementioned publication suggests to detect the position of the contact surface of the contact glass by means of an interference arrangement. However, the interference patterns produced thereby are generally not suitable for adjustment or consideration by inexperienced users. They are extremely difficult to evaluate automatically. The concept of said citation, as far as it is suitable for application, requires either irradiation of high-energy laser radiation forming optical breakthroughs at the boundary surface of the contact glass, which is disadvantageous under aspects of radiation protection, or requires contact glass materials which exhibit a non-linear effect for the processing laser radiation.
In contact elements, the contact surface to be placed on the object is usually manufactured with great precision. Therefore, the aforementioned methods and devices of WO 04/032810 A2 determine the position of the contact surface, which may be planar, for example.
Therefore, it is an object of the invention to improve a method or device of the above-mentioned type with respect to determining the position of the contact glass.